In order to meet the increasing demands for wireless data traffic, wireless communication systems have been developed to support a higher data rate. For an increase in the data rate, the wireless communication systems have been evolved to improve the spectral efficiency based on the communication technologies such as Orthogonal Frequency Division Multiple Access (OFDMA) and Multiple Input Multiple Output (MIMO). However, the tremendous demands for wireless data traffic may not be met only with the improvement of the spectral efficiency.
Recently, the increase in demands for smart phones and tablet computers and the explosive growth in the number of applications requiring a large amount of traffic have accelerated the demands for data traffic. A method for meeting these demands is to use more frequency resources over a wider frequency band. Therefore, in order to secure a wider frequency band and apply the wider frequency band to wireless mobile communication, there is a need to consider securing an ultra-wideband frequency in the higher-frequency domain.
Wireless communication in the millimeter Wave (mmWave) band may suffer from an increase in the propagation loss such as path loss and return loss due to the frequency characteristics of the mmWave band. Because of the increase in the propagation loss, the arrival distance of radio waves is reduced causing a reduction in coverage. On the other hand, the wavelength may be significantly reduced due to the mmWave band, so it may be easy to apply beamforming that uses a plurality of small antennas. Accordingly, it is possible to seek a new way to increase the arrival distance of radio waves and the coverage by mitigating the path loss of radio waves by applying the beamforming technology to the wireless communications in the mmWave band.
Transmit beamforming is generally a method of increasing directivity by concentrating arrival areas of radio waves in a specific direction using a plurality of antennas. A set of multiple antennas is called an antenna array, and each antenna included in the antenna array is called an array element. The antenna array may have a variety of types, including a linear array and a planar array. With the use of the transmit beamforming, the transmission distance may be increased due to the increase in directivity of signals, and signals are not transmitted in directions other than a specific direction, so signal interference to users other than the user associated with the specific direction may be significantly reduced.
A receiving side may also perform beamforming on received signals using a receive antenna array. A receiver may increase the sensitivity of signals received in a specific direction by concentrating received radio waves in the specific direction, and blocking interference signals by excluding signals received in directions other than the specific direction from the received signal.
In a UL of the existing cellular system, an operation is performed to adjust UL transmit timing and power of a Mobile Station (MS) to match with receive timing and receive dynamic range of a Base Station (BS) using UL Ranging (RNG) or Random Access Channel (RACH) at and/or after initial network entry. In the existing cellular system that basically considers equi-directional or omni-directional transmission of MSs, focuses have been made on the adjustment of transmit timing and/or power using UL RNG or RACH, and its associated protocol and signaling technologies have been mainly developed.
In contrast, as to the recent prior arts in the beamforming-based cellular system, technology for transmitting beam selection information for a Downlink (DL) and the transmission procedure that considers beamforming during initial network entry have been mainly researched, but there has been little development in the technology for supporting UL beamforming.